The present invention relates to a semiconductor device fabricated with an organic material having bi-polar charge-transport characteristics.
The metal-oxide semiconductor field-effect transistor (MOSFET or MOS device) is a dominant and important device in fabricating memory devices and integrated circuits, and various types of MOSFETS are known. MOSFET technology includes NMOS, PMOS, and CMOS technology. NMOS and PMOS devices are n-channel and p-channel devices, respectively, and CMOS devices comprise n-channel and p-channel devices integrated on the same chip. Other acronyms used to identify MOSFETs include DMOS (wherein xe2x80x9cDxe2x80x9d stands for xe2x80x9cdiffusionxe2x80x9d or xe2x80x9cdouble diffusionxe2x80x9d) , IGBT (Insulated Gate Bipolar Transistor), BCMOS (CMOS having bipolar devices), and DGDMOS (Dual Gate DMOS).
Demands for low-cost consumer electronics (e.g. large emissive displays, electronic paper, smart cards, and so forth) have precipitated the development of organic materials suitable for use in field-effect transistors (FETs) and other devices as conducting, semiconducting, and light-emitting materials. Organic materials are attractive for use in electronic devices as they are compatible with plastics and can be easily fabricated to provide low-cost, lightweight, and flexible devices with plastic substrates. Organic materials having a conductivity and carrier mobility permitting their use as the active layer in thin-film semiconductor devices are described in U.S. patent application Ser. No. 08/770,535, filed Dec. 20, 1996, xe2x80x9cMethod of Making An Organic Thin Film Transistor,xe2x80x9d by Zhenan Bao et al. (an inventor herein) (hereinafter the xe2x80x9cBao ""535 applicationxe2x80x9d), assigned to the present assignee and incorporated herein by reference. An organic device is further described in U.S. patent application Ser. No. 09/087,201, filed May 29, 1998, xe2x80x9cThin-Film Transistor Monolithocally Integrated With an Organic Light-Emitting Diode,xe2x80x9d by Zhenan Bao et al. (an inventor herein), which also is assigned to the present assignee and incorporated herein by reference.
Nearly all currently known organic materials useful as semiconductors in field-effect transistors are uni-polar, meaning that they are either p-type (positive charge transporting) or n-type (negative charge transporting). The uni-polar characteristics of these materials complicates the fabrication process and affects the cost of the FETs, because uni-polar materials must be patterned to form the desired regions of the FET device. For example, a CMOS device includes both p-type and n-type channel regions, and when uni-polar materials are used, these regions must be separately patterned and deposited. A significant cost reduction would ensue if bi-polar materials could be used, particularly with CMOS devices. Bi-polar semiconducting materials would not require patterning or at least require less patterning than uni-polar materials. A CMOS device is an advantageous circuit configuration for organic semiconductors, because it has a low static power dissipation and the transistors are xe2x80x9conxe2x80x9d only during switching.
Additionally, there has been great interest recently in developing electronic circuits using fabrication methods that rely upon high-performance solution-processable materials, such as screen printing and ink-jet printing. Presently, of the uni-polar organic semiconductors, only a limited number are capable of operating in air, and the availability of soluble, semiconducting polymers remains limited. See, e.g., Bao, Z. et al., xe2x80x9cSoluble and Processable Regioregular Poly(3-Hexylthiophene) for Thin Film Field-Effect Transistor Applications with High Mobilityxe2x80x9d, APPL. PHYS. LETT., Vol 69, No. 26, at pp. 4108-4110 (1996), which is incorporated herein by reference. It would be advantageous to provide a device with an active semiconducting layer comprising a solution processable thin film.
As may be appreciated, those in the field of semiconducting devices continue to search for new materials and components to reduce the size, increase the efficiency, simplify the fabrication, and reduce the cost of fabricating the devices. In particular, it would be advantageous to provide a semiconducting device including use of a bi-polar organic material that is solution processable. These and further advantages may appear more fully upon considering the description given below.
Summarily described, the invention embraces a field-effect transistor comprising an organic semiconducting layer having bi-polar charge transport characteristics. The semiconducting layer comprises a bi-polar polymeric film effective for hole and electron transport comprising a polymer having a conjugated framework with functional moieties capable of solvating ions or promoting ionic charge transport. The conjugated framework is selected from at least one of thiophene, pyrrole, benzene, naphthalene, antrhacene, and antrhacene-dione, and the functional moieties are selected from (i) functional side groups comprising salts of carboxylic acid and sulfonic acid and (ii) functional sites selected from heteroatoms having electron lone pairs comprising sulfur, nitrogen, and oxygen. The field-effect mobility of the bi-polar polymeric film is at least 10xe2x88x923 cm2/Vs when operating as an n-type or p-type device.